Previously, we found that the fat body of Aspisoma lineatum Gyll (Coleoptera: Lampyridae) firefly larvae is weakly bioluminescent. This tissue is very different from that of other insect larvae. It is macroscopically distinguished by its color (pinkish and whitish), morphology, and the absence of oenocytes. It is composed of trophocytes that are arranged in groups of globular units covered by a layer of basal lamina. The cytochemistry indicated that the trophocytes have glycoproteins, which are produced by a well-developed rough endoplasm reticulum (RER). Expanded RER cisterns indicated intense protein synthesis by the trophocytes. Lipid droplets are also present in the trophocytes. Charge-coupled device imaging showed that the fat body produces a continuous bioluminescence whose intensity is 2–3 orders of magnitude lower than that of the lanterns, a result that is explained by the lower contents of luciferin and luciferase in the fat body compared with the lanterns. Expression of different luciferase isozymes in the fat body and lanterns is confirmed by bioluminescence spectral and kinetic analyses. Trophocytes were identified as the emitting cells, suggesting that the larval and adult lantern's photocytes may have evolved from fat body trophocytes.